EFFECT OF NON-PROPORTIONAL OVERLOADING ON FATIGUE CRACK GROWTH

The effect of proportional and non-proportional overloading on mode l fatigue crack growth have been studied,and the influences of crack tip plastic zone,crack tip blunting as well as crack closure were discussed.Proportional(model I)overloading may cause more serious crack growth retardation than non-proportional(mixed mode)overloading.Therefore,for estimating the fatigue life of engineering structures to simplify a real overload which may of- ten be non-proportional as a proportional one is not always safe.

THE THREE-DIMENSIONAL STRESS INTENSITY FACTOR UNDER MOVING LOADS ON THE CRACK FACES

The dynamic stress intensity factor history for a half plane crack in an otherwise unbounded elastic body,with the crack faces subjected to a traction distribution consisting of two pairs of combined mode point loads that move in a direction perpendicular to the crack edge is considered.The analytic expression for the combined mode stress intensity factors as a function of time for any point along the crack edge is obtained.The method of solution is based on the application of integral transform together with the Wiener-Hopf technique and the Cagniard-de Hoop method. Some features of the solution are discussed and graphical results for various point load speeds are presented.

Structural analysis and design of frost resistance function for subgrade of high-speed railway ballasted track in cold regions

According to the technical characteristics of short fixed wheelbase of a high-speed carriage, a subgrade-track integrated space mechanical response analysis model is proposed for trains under the action ofbiaxial load after the comparison of the stress distribution characteristics of the ballast track subgrade bed structures for high-speed railway under the action of uniaxial load and biaxial load. The loading threshold value （high-cycle long-term dynamic strength） under the circum- stance where the cumulative deformation of subgrade structure gradually develops and finally reaches the convergent state, and its relationship with the foundation coefficient K30 were deduced, based on the characteristics of cumulative defor- mation evolution obtained from the unit structure filling model test under the action of cyclic loading. In view of structure stability and frost resistance requirements of the railway subgrade in cold regions, technical conditions to maintain good service performance of subgrade structure of high-speed railway ballasted track are discussed and analyzed. Study results show that the additive effect manifests itself obviously for railway train bogies under the action of biaxial load than uni- axial load, which has a significant dynamic effect on the subgrade bed bottom and a slight effect on the surface layer. Thus, the adoption of a biaxial load model in the design of a high-speed railway subgrade accurately reflects the vehicle load. Pursuant to the structure design principle, the design method of the subgrade structure of high-speed railway ballasted track is proposed to meet the technical requirements such as structural strength, bearing stiffness and high-cyclic and long-term stability. Technical indicators are obtained for the variation of thickness of the surface layer of reinforced sub- grade bed in the double-layer subgrade mode along with the change of K30 at the subgrade bed bottom. The double-layer structure mode of ＂closure on the upper layer and drainage on the lower layer＂ was proposed in order to meet the water- proofing and drainage requirements of the upper layer of the subgrade bed in cold regions. A dense-framework graded gravel filler with weak water permeability at a coefficient of 10 4 cm/s is used on the upper layer and the void-framework graded gravel filler at the water permeability coefficient of 10 2 cm/s is adopted on the lower layer.

Influence of stress path change on the resistance to plastic deformation of cold rolled sheets

Flat workpieces have been tested in order to investigate the influence of stress path change （loading mode） while keeping strain path unchanged. These investigations are pertinent to the testing of cold rolled strips and to subsequent forming. The workpieces which first compressed by plane strain compression in thickness direction were then tested in perpendicular direction in order to measure the influence of strain and stress path. The tension workpieces came from flat die compression test at different deformation histories. Two different materials were investigated： 18/8 Ti stainless steel and AW-1050 aluminium. The results show that the plastic flow by tension in lengthwise direction after pre-strain by compression in thickness direction will begin at an appreciably lower stress than that of the workpieces unloaded after pre-compression. Comparing with two materials, it can be seen that both 18/8 Ti stainless steel and AW-1050 aluminium behave similarly. The drop in yield stress is lower for AW-1050 aluminium than that for 18/8 Ti stainless steel. However, reloading in different directions than in the precious step results in significantly higher strain hardening.

Analysis of composite material interface crack face contact and friction effects using a new node-pairs contact algorithm

A new node-pairs contact algorithm is proposed to deal with a composite material or bi-material interface crack face contact and friction problem （e.g., resistant coating and thermal barrier coatings） subjected to complicated load conditions. To decrease the calculation scale and calculation errors, the local Lagrange multipliers are solved only on a pair of contact nodes using the Jacobi iteration method, and the constraint modification of the tangential multipliers are required. After the calculation of the present node-pairs Lagrange multiplier, it is turned to next contact node-pairs until all node-pairs have finished. Compared with an ordinary contact algorithm, the new local node-pairs contact algorithm is allowed a more precise element on the contact face without the stiffness matrix singularity. The stress intensity factors （SIFs） and the contact region of an infinite plate central crack are calculated and show good agreement with those in the literature. The contact zone near the crack tip as well as its influence on singularity of stress fields are studied. Furthermore, the frictional contacts are also considered and found to have a significant influence on the SIFs. The normalized mode-II stress intensity factors KII for the friction coefficient decrease by 16% when f changes from 1 to 0.

Bridging the gap between laboratory and field moduli of asphalt layer for pavement design and assessment:A comprehensive loading frequency-based approach

Asphalt pavement is a key component of highway infrastructures in China and worldwide.In asphalt pavement design and condition assessment,the modulus of the asphalt mixture layer is a crucial parameter.However,this parameter varies between the laboratory and field loading modes(i.e.,loading frequency,compressive or tensile loading pattern),due to the viscoelastic property and composite structure of the asphalt mixture.The present study proposes a comprehensive frequency-based approach to correlate the asphalt layer moduli obtained under two field and three laboratory loading modes.The field modes are vehicular and falling weight deflectometer(FWD)loading modes,and the laboratory ones are uniaxial compressive(UC),indirect tensile(IDT),and four-point bending(4PB)loading modes.The loading frequency is used as an intermediary parameter for correlating the asphalt layer moduli under different loading modes.The observations made at two field large-scale experimental pavements facilitate the correlation analysis.It is found that the moduli obtained via laboratory 4PB tests are pretty close to those of vehicular loading schemes,in contrast to those derived in UC,IDT,and FWD modes,which need to be adjusted.The corresponding adjustment factors are experimentally assessed.The applications of those adjustment factors are expected to ensure that the moduli measured under different loading modes are appropriately used in asphalt mixture pavement design and assessment.

Numerical Research on the Cold Start-up Strategy of a PEMFC Stack from-30℃

The cold start-up of the PEMFC(proton exchange membrane fuel cell)stack from sub-zero temperature is considered one of the significant obstacles to its expansive commercial applications.In the cold start-up process,with the progress of hydrogen/oxygen chemical reaction,the produced water will freeze into ice,occupying the pores of the porous electrode,thus leading to a rapid deterioration of output performance and even making the cold start-up fail.In this work,a one-dimensional numerical model is adopted to study a cold start-up process of the PMEFC stack starting from-30℃.The stepwise-changed current loading mode is employed in the process.An assisted preheating method is used to explore an optimal operating condition for a successful cold start-up.The numerical results are validated by comparing the numerical result with the experimental data in reference,and they agree with the experimental data very well.The results show that the optimal heating power in the studied range is 100 W.As the initial current slope increased,the current peak value increased,but the cold start-up process failed.Also,the start-up time and ice volume fraction are highly dependent on the initial current slope.The optimal initial current slope is 0.7 A/s.Besides,a higher initial current slope will cause a significant inner ohmic resistance.The resistance of CLa(catalyst layer of the anode)is the key and primary part of the total ohmic resistance.The details of the research and the analyzed results will help design the cold start-up strategy for the PEMFC stack start-up from-30℃.